1. Field of the Invention
The present invention relates to a phase change optical recording medium.
2. Description of the Related Art
There are two types of optical recording medium on which data can be recorded, reproduced, and deleted by irradiation of a semiconductor laser beam. One is a magneto optical medium on which data are recorded (or written) and deleted (or erased) by reversing magnetic polarity, and the other is a phase change medium on which reversible phase change between a crystalline phase and an amorphous phase is used for recording and deleting data. The latter is characteristic in that it is capable of repetitive recording by a single beam and its disk drives have simpler optical mechanisms, and it has been applied to a recording medium in the fields related to computers and audio visual systems.
Information is written on and erased from a phase change optical recording medium by irradiating a laser light beam on a thin film of recording layer of the medium, thereby heating the recording layer so that its structure changes from a crystalline phase to an amorphous phase or vice versa, changing its reflectivity of the disk. Typically, the entire surface is crystalline in an initial state and data that are formed of amorphous marks and crystalline spaces are recorded onto the surface.
A typical method by which amorphous marks are made on a phase change disk is shown in FIG. 1. By modulating laser power to three values, writing power Pw, erasing power Pe, and bias power Pb (Pw>Pe>Pb), writing and erasure are carried out. In order to make marks, a pulse train of Pw and Pb pulses is irradiated. A Pw pulse melts the recording layer, after which the layer rapidly cools as the laser power is modulated to Pb, and the molten area becomes an amorphous phase. The laser power is then changed to Pe to crystallize the rear edge of the amorphous section and form a mark having a predetermined length. To conduct a repetitive recording, or overwrite, a so-called direct overwrite is possible. In the direct overwrite, a modulated laser beam is applied in the same manner on a track that is already recorded to form new marks at predetermined positions while crystallizing and deleting old amorphous marks that have been recorded simultaneously.
The simplest structure of a phase change optical recording medium has, on a transparent substrate, a first protective layer, a recording layer, a second protective layer, and a reflective layer disposed in this sequence, or in the opposite sequence, to which a laser beam is irradiated from the side of the first protective layer to record and reproduce data. Typically, a mixture of ZnS and SiO2 is used for the first and second protective layers, a mixture of Ag, In, Ge, Sb, Te, and the like for the recording layer, and an Al alloy or an Ag alloy for the reflective layer. When an Ag alloy is used, a sulfuration preventive layer may be introduced between the second protective layer and the reflective layer. Although it may differ depending on the composition of the layers, an optical recording medium of such structure will deteriorate and cannot be used any further after several hundreds to several thousands of overwrites since jitter gradually increases as the medium is repeatedly overwritten.
It has been suggested, therefore, to use a material that includes ZrO2 for a part or all of the protective layers, which are adjacent to the recording layer, to improve durability to repetitive recording. The suggestion is based on the recognition of excellent heat resistance and mechanical strength of ZrO2. However, sufficient durability to repetitive recording cannot be achieved for a wide range of recording power only by incorporating a structure in which a material including ZrO2 is adjacent to the recording layer. Particularly, it is not possible to reduce jitter to a satisfactory level when the recording power is low.
For example, in inventions disclosed in Japanese Patent Application Laid-Open (JP-A) No. 05-144085, JP-A No. 08-180458, JP-A No. 2000-348380, and JP-A No. 2000-182277, optical and thermal conditions are not optimized and therefore the inventions cannot be used to manufacture an optical recording medium durable to repetitive recording for a wide range of recording power.
In an invention disclosed in JP-A No. 11-339314, a layer including ZrO2 is arranged adjacently to a recording layer, but the ZrO2 layer is used only at the interface to a first protective layer.
An invention disclosed in JP-A No. 08-96411 describes an information-recording medium having a dielectric protective layer and a light-absorbing protective layer between a recording layer and a reflective layer. The object of the invention, however, is to manufacture an information-recording medium suitable for high-density recording by reducing the difference of light absorbance between an amorphous state and a crystalline state of the recording layer.